1. Field of the Invention
This invention relates to an apparatus for measuring differential pressure between points in a fluid passage. More particularly, the invention relates to an apparatus for minimizing the affect of a short term pressure fluctuation upon the output of a differential pressure sensor in communication with a first point and a second point in a fluid line.
2. Description of Prior Art
During the operation of many fluid systems, it is common practice to measure the pressure difference between two points in the fluid conducting passage, one point being upstream with respect to the other point. Oftentimes the fluid moving in the passage is subjected to pressure fluctuations of short time duration which tend to distort any measurement of the effective differential pressure between the points being observed. As for example, during the drilling of oil or gas wells, it is common to drill the well with a rotary bit suspended on the end of drill pipe in the wellbore. A drilling fluid, such as mud, is circulated down the drill pipe to the bit and back up the annulus. Since the borehole conditions are continually changing as the bit proceeds through various formations, it is desirable to continually monitor the rate the mud is being pumped into the wellbore. This flow rate is proportional to the differential pressure between two points in the mud line and thus the output of a differential pressure sensor can be used as a flow meter.
It has also been found that changes in the borehole condition can be effectively monitored by the use of an analog system whereby an adjustable flow restrictor is placed in the mud line and the pressure loss across the restrictor is adjusted to analogize the pressure loss of the mud through the line and bit. Thus, changes in borehole conditions which effect the pressure of the mud system can be detected by a comparison of the changes in the differential pressure across the flow restrictor and the differential pressure between the input and output mud flow lines of the well. A more complete disclosure of this analog system is given in my previously issue U.S. Pat. No. 3,595,075.
Many fluid conducting systems are characterized by short term pressure fluctuations which create erratic and often erroneous pressure measurements in a differential pressure sensor. As for example, in the drilling of oil and gas wells the pump used to force the mud through the flow lines, to the bit and back up the borehole is typically of the piston and cylinder type of positive displacement pump. Thus the fluid pressure in that portion of the input mud line near the pump is not constant but pulsates with each stroke of the pump. Even though such fluctuations or surges do reflect proportional changes in the mud flow rate within the line, the mud capacity of the drill string and the borehole annulus smooth out such surges in the borehole so that the acutal mud flow in the borehole is at an approximately constant rate proportional to the average pump output. Accordingly, the operators of such drilling rigs wish to observe the average mud flow rate as reflected by the average pressure differential between the two points being measured. This requires that the operator be able to observe slight variations in the average pressure differential unhampered by the fluctuations created by the reciprocal pump.
Typically in oil well drilling operations the mud pump produces a fluid pressure in the range of 2000 p.s.i. at the first or upstream point being observed. Since the mud pump is of the reciprocating type, a pressure fluctuation is produced upon each stroke, typically of the order of 200 p.s.i., with the mud line pressure varying from 1800 p.s.i. to 2200 p.s.i. This 400 p.s.i. variation does not create an identical 400 p.s.i. variation at the second or downstream point because the differential pressure created between the two points by frictional loss is related to flow rate which in turn is related to pressure. Accordingly, if there exists a 10 p.s.i. differential between the first and second points at a constant 2000 p.s.i. pressure, as the pressure decreases to 1800 p.s.i. at the first point, the pressure at the second point would decrease to 1,789 p.s.i. because the differential pressure also changes a full 10%. Since the initial differential pressure was 10 p.s.i., the 1 p.s.i. change is a 10 % error induced solely by pressure fluctuations.
In the past, in order to facilitate the accurate measurement of the pressure differential between two points in a fluid conduit, it has been thought that the pressure fluctuations could be dampened out of the fluid conduit upstream of the two points being monitored or dampened out as they appear in the signal lines leading to the differential pressure sensor.
There have been many attempts to dampen out these surges and minimize the resultant differential pressure errors caused by these fluctuations. Such attempts have generally taken the form of utilizing a pair of matched small cross-sectional area restrictors placed in each of the lines leading to the differential pressure sensor. These restrictors dampen the rapid pressure fluctuations and transmit long term changes representative of the true changes in the average flow rate. However, in practice it is very difficult to use and maintain such restrictors due to the difficulty in making a perfectly matched pair. Even when one or both restrictors are made adjustable so that a theoretically perfect match can be made, such restrictors do not have equal operating characteristics for flow in each direction. Therefore, when the fluid pressure or fluctuation rate changes either restrictor may fail to produce a true average pressure and thereby create an error in the differential pressure measurement.
In addition to the use of restrictors, attempts have been made to use various accumulator tanks or other volume containers to dampen pressure fluctuations either directly in the fluid conduit or in the lines leading to the differential pressure sensor. Generally, all such attempts have not effectively eliminated the effect of such fluctuations on a differential pressure sensor. Many such devices use an elastic bladder or ball fluid with gas enclosed within a rigid chamber. Such a device is typically connected directly into the fluid line where it dissipates the energy of the pressure surges by allowing the gas filled bladder to compress or collapse. Such devices are disclosed in U.S. Pats. issued to Zahid No. 3,782,418; Mercier No. 2,773,455 and to Kupiec No. 3,550,634. Other patents relating to eliminating pressure surges are Hollander No. 2,187,972; Hollander No. 2,239,715; Mayer No. 2,239,727; Reed No. 2,595,540; Crookston No. 2,811,925 and Wilson No. 2,934,025.